Fluoride is a finicky friend to teeth. Too little of it, and you get cavities. Too much, and it starts to eat away and discolor the enamel of your pearly whites.

The federal government said Friday morning that the fluoride seesaw in this country has tipped too far toward excess. These days kids are getting fluoride from many sources, including drinking water, toothpaste and mouth rinses.

So the U.S. Department of Health and Human Services together with the U.S. Environmental Protection Agency are proposing to drop the recommended level of fluoride in drinking water to the lowest end of the current range. That would put it at 0.7 milligrams per liter of water (mg/L) quite a bit below the previous recommended maximum of 1.2 mg/L.

The move follows a study from the U.S. Centers for Disease Control and Prevention that found that 2 of 5 adolescents have tooth streaks or spots because of excess fluoride. The condition, called dental fluorosis, usually develops when children's teeth are forming — age 8 and younger.

Dental fluorosis can happen after drinking artificially fluoridated water or water naturally high in fluoride. HHS says that in the United States fluorosis is usually very mild or mild, with "barely visible lacy white markings or spots on the enamel." In other countries with higher levels of naturally occurring fluoride in water, severe fluorisis can pit teeth and turn them brown.

HHS and EPA's decision to scale the chemical back was also based on findings of a 2006 report from the National Academy of Sciences, which found that the EPA standard was leading to the loss enamel in some children.

Despite the new concern over fluoride, public health officials says fluoridated water and fluoride-fortified toothpaste are largely responsible for the significant decline in tooth decay in the U.S. over the past several decades.

And some dentists are even skeptical that the new limits are necessary. Shots spoke to John Liu, president of the American Academy of Pediatric Dentistry, who said that he's concerned that the announcement is going to give parents the impression that fluoridated water is now unhealthy for kids.

"There's a cosmetic risk, not a health risk" if kids consume a lot of fluoride, Liu said. And, he added, that excess fluoride is more likely to come from too much toothpaste or mouth rinses than water, since kids don't typically drink a lot of it.

Shots is the online channel for health stories from the NPR Science Desk. We report on news that can make a difference for your health and show how policy shapes our health choices. Look to Shots for the latest on research and medical treatments, as well as the business side of health. Your hosts are Scott Hensley and Nancy Shute. You can reach the Shots team via our contact form.